Work Process and Memory Management in SAP

Let’s talk about the entire concepts that are related to memory when we talk about SAP Application.

Starting with few basic terminologies,

Local Memory : 

Local process memory, the operating system keeps the two allocation steps transparent. The operating system does the other tasks, such as reserving physical memory, loading and unloading virtual memory into and out of the main memory.

Shared Memory : 

If several processes are to access the same memory area, the two allocation steps are not transparent.

One object is created that represents the physical memory and can be used by various processes. The processes can map the object fully or partially into the address space. The way this is done varies from platform to platform. Memory mapped files, unnamed mapped files, and shared memory are used. 

Extended Memory :

SAP extended memory is the core of the SAP memory management system. Each SAP work process has a part reserved in its virtual address space for extended memory. You can set the size of extended memory using the profile parameter em/initial_size_MB: Extended Memory Pool Size.

Mapping can be done from any extended memory from the common resource to any work process.

[If you have programming understanding , It is majorly used when pointers are involved in ABAP Program]

SAP Swap Memory :

Allocation of memory for the current internal session by transferring pages out of memory, similarly to operating system paging. SAP Paging enables the roll area to be extended at ABAP runtime when a large dataset, internal tables is handled.

Virtual Memory :

Virtual memory is fully independent of the physical main memory.

All operating systems which we use for SAP Applications support virtual memory technology. 

A process allocates virtual memory using logical (virtual) addresses. Each process has its own virtual address space 

Lets understand what is Address Space, The address space consists of a text segment, a data segment, a dynamically extendible heap, and a dynamically extendible stack. 

About Text segment (a.k.a code segment) and data segment. 

Text segment is where you store the program.

Data segment is where you save the data.

About Heap and Stack Memory Allocation :

The heap increases in size from the bottom , The memory is allocated during the execution of instructions written by programmers.

The stack increases from the top and  allocation happens on contiguous blocks of memory, which enables the entire virtual address space to be used. 

This also applies to SAP work processes

Understand it this way , 

With 32-bit systems, 4 GB of memory is theoretically available. Depending on the operating system, there is approximately 2 GB of virtual address space available to a process.

We reserve heap area for SAP Work Processes (we have profile parameters to set them up)

1. SAP Roll Area : The roll area is area in memory which belongs to a work process(we can configure that ). It is located in the heap of the virtual address space of the work process.

When the context of a work process changes (when the previous work process is completed and new one task take up to that work process ), the data is copied from the roll area to a common resource known as the roll file. To prevent repeated copying, another roll buffer is located in between, which is part of the shared memory.

We have two segments in roll area, 

ztta/roll_first : assigned to the work process first as memory

ztta/roll_area : if the memory that was assigned earlier is used and the work process needs more memory then this memory is assigned.

The amount of memory is determined by the difference between the ztta/roll_area and ztta/roll_first parameters.

Roll area holds data from previous dialog steps and data that characterize the user.

 

2. SAP Paging Area :  The paging area stores special data for the ABAP processor, while the extended memory stores a large portion of the internal tables of a program.

Parameter which determines this is rdisp/PG_MAXFS

Paging area holds data from the application programs.

What is stored in Paging Area here is an example :

    The ABAP/4 runtime system and the ABAP/4 compiler use a common                    interface to store different types of data in different parts of the SAP paging             area. This data includes the ABAP/4 memory (EXPORT TO MEMORY), the                    SUBMIT REPORT parameters, CALL DIALOG and CALL TRANSACTION                        USING, as well as internally defined macros (specified with DEFINE).

We have a issue mentioned related to SAP Paging which clears more about Paging : Read Here

3. Private memory : This is nothing but private heap memory as we give the most priority to dialog process we reserve heap memory for the dialog process. After releasing the assigned memory (difference of roll_first and roll_area ), the operating system still considers the (virtual) memory as being occupied by the allocating process. 

The work process can be run in PRIV mode (private) when the local memory is assigned, The work process is reserved for processing the current user context until the context releases the work process again when the request has ended. 

Up to this time point no other user context can be mapped to the work process.

Once the user context is terminated (work process is released, restart : restart occurs when the work process use more local memory than is defined in the parameter abap/heaplimit ) , local memory is returned. 

If a dialog work process has used up the roll area assigned to it and the extended memory, private memory is assigned to the work process. The work process goes into PRIV mode.

In the parameter settings, ensure that not too many dialog work processes are simultaneously in the PRIV mode, since this lowers system performance.

The other work process types , the non dialog work process  (background, update, enqueue and spool work processes) are assigned heap memory after the roll area is used up. Allocating Extended Memory is not useful since no context change occurs for these processes.

Since we are on this topic , lets talk about Automatic Termination of PRIV mode Processes :

If too many dialog work processes run in the PRIV mode in an SAP application server, performance problems arise. The normal processing of user inquiries is slowed down by the PRIV mode work processes.

The SAP system offers a way out that lets you terminate non-active dialog processes in the PRIV mode. 

We set rdisp/wppriv_max_no , say n . What happens after this is if you have more than this number of work process running in private mode. Work processes are active and the time span set in parameter rdisp/max_priv_time has elapsed, the transaction for that PRIV process, which has spent the longest possible time in PRIV mode, is reset

Note :-

rdisp/max_priv_time (default 600 seconds)

rdisp/wppriv_max_no. (Number of dialog work processes - 5 or 1 which ever has the grater value among this two )

Example :- 

If we set rdisp/wpriv_max_no = 5 and rdisp/max_priv_time = 600 second. Then, the SAP system checks periodically if more than 5 dialog work processes are in PRIV mode after 600 seconds (10 minutes). If this is the case, the transaction is reset that has been in PRIV mode for the longest time

Virtual Address Work Space 

 Image Source

What is Memory Allocation ?

Reserving a segment in physical memory , it makes more sense when you take the image as reference. Linking the physical memory segment to the virtual address space of the process, which means reserving a segment of the same size in the virtual address space and mapping the virtual addresses to physical addresses.

Function of SAP Memory Management

An application runs in an SAP work process where an ABAP program is normally executed. The process requires memory to do this, which is allocated to the process by the memory management system.

We have separate way of dealing with Dialog Work Process and Non Dialog Work Process.

This would be the right time for you to understand few more terminologies:-

User Context : 

Temporary memory area that contains information about users which is used throughout the program processing. User information that is stored [Current Settings of the user ,  User Authorizations , Names of program that user are currently using] , Once they log off this memory is released.

The user context area that allows direct access can be extended as required when the size of the user context increases. For dialog work processes, the data of the user context, including internal tables is located in this expanded area. You can therefore access all the data in the user context. Only data of types extract and export to memory still remain in SAP Paging

Roll Area : 

Memory area which is allocated by work processes for the program instance.

Each time a user starts a program, a roll area is created for that instance of the program.Program Information contained [ Dynamic Memory Allocations, Variable Value and Current pointer of the program ] 

The memory in the roll area becomes free when the program ends its execution.

If two users run the same program at the same time then two roll areas will be created one for each user.

The SAP Roll Area is used for the initial memory assigned to a user context, and (if available) for additional memory if the expanded memory is full.

Roll-in: cross-user data is rolled in from the common resource in the work process (and is processed there).

Roll-out: User-specific data is rolled out from the work process in the common resource (after the dialog step has ended)

Image Source (help.sap.com)

Execution of Work Process ->wp in case of Dialog and background Job

In case of Dialog work Process

In case of Background Process

At first Roll_area is used : Based on Parameter Ztta/roll_first. After that we use the extended memory that is mentioned in em/intial_size_MB and use quota for particular wp which is mentioned in ztta/roll_extension_dia. After that the left roll area is used (the memory can be calculated by calculating the difference between ztta/roll_area and ztta/roll_first) Post that we use private memory [Local Heap Memory]   Limit of heap memory are configured using these parameters: abap/heap_area_dia abap/heap_area_total OS Limits allocation of Memory , swap space in the host system is used up or the upper limit of the OS address space is reached.

Similarly roll area is used “ztta/roll_area”  Unlike Dialog work process extended memory are not used right now  Roll area is already used in this scenario Heap area is used , Unlike Dialog Work process Private Memory [Local Heap memory] is out of scope.  Either limit of heap memory is used based on the SAP parameter “abap/heap_area_nondia” or it uses entire heap memory assigned for all the work process and its value based on SAP parameter “abap/heap_area_total” OS limits allocation memory. SWAP space is completely used up, Rare to occur. 5. If still required is full then the extended memory is used.

Graphical Representation 

In case of Dialog process :-

In case of Non Dialog Process :-

Note :-  Once work process comes in PRIV mode it  cannot be terminated by the parameter “rdisp/max_wprun_time”, it remains connected to the user until the user ends the transactions or it is timed out by the system due to the memory or explicitly logged out using SM04/DPMON.

Read More :-

help.sap link

SAP Blog by Samar

SAP User Context and Roll Area

Working model Example

SAP Memory Management (BC-CST-MM) - SAP Library

SAP Memory Management Implementations (In Process) - Application Server Infrastructure - SCN Wiki

Memory Parameters - SAP NetWeaver Business Warehouse - SCN Wiki

Cracking SAP Memory Management _Part1

Cracking SAP Memory Management _Part2

Cracking SAP Memory Management _Part3

Memory Allocation and Analysis - Application Server Infrastructure - SCN Wiki